gofmt

Documentation

Overview

Gofmt formats Go programs. It uses tabs for indentation and blanks for alignment. Alignment assumes that an editor is using a fixed-width font.

Without an explicit path, it processes the standard input. Given a file, it operates on that file; given a directory, it operates on all .go files in that directory, recursively. (Files starting with a period are ignored.) By default, gofmt prints the reformatted sources to standard output.

Usage:

gofmt [flags] [path ...]

The flags are:

-d
	Do not print reformatted sources to standard output.
	If a file's formatting is different than gofmt's, print diffs
	to standard output.
-e
	Print all (including spurious) errors.
-l
	Do not print reformatted sources to standard output.
	If a file's formatting is different from gofmt's, print its name
	to standard output.
-r rule
	Apply the rewrite rule to the source before reformatting.
-s
	Try to simplify code (after applying the rewrite rule, if any).
-w
	Do not print reformatted sources to standard output.
	If a file's formatting is different from gofmt's, overwrite it
	with gofmt's version. If an error occurred during overwriting,
	the original file is restored from an automatic backup.

Debugging support:

-cpuprofile filename
	Write cpu profile to the specified file.

The rewrite rule specified with the -r flag must be a string of the form:

pattern -> replacement

Both pattern and replacement must be valid Go expressions. In the pattern, single-character lowercase identifiers serve as wildcards matching arbitrary sub-expressions; those expressions will be substituted for the same identifiers in the replacement.

When gofmt reads from standard input, it accepts either a full Go program or a program fragment. A program fragment must be a syntactically valid declaration list, statement list, or expression. When formatting such a fragment, gofmt preserves leading indentation as well as leading and trailing spaces, so that individual sections of a Go program can be formatted by piping them through gofmt.

Examples

To check files for unnecessary parentheses:

gofmt -r '(a) -> a' -l *.go

To remove the parentheses:

gofmt -r '(a) -> a' -w *.go

To convert the package tree from explicit slice upper bounds to implicit ones:

gofmt -r 'α[β:len(α)] -> α[β:]' -w $GOROOT/src

The simplify command

When invoked with -s gofmt will make the following source transformations where possible.

An array, slice, or map composite literal of the form:
	[]T{T{}, T{}}
will be simplified to:
	[]T{{}, {}}

A slice expression of the form:
	s[a:len(s)]
will be simplified to:
	s[a:]

A range of the form:
	for x, _ = range v {...}
will be simplified to:
	for x = range v {...}

A range of the form:
	for _ = range v {...}
will be simplified to:
	for range v {...}

This may result in changes that are incompatible with earlier versions of Go.

Notes

Bugs

• The implementation of -r is a bit slow.

• If -w fails, the restored original file may not have some of the original file attributes.